The invention relates to synchronous machines, in particular permanent magnet synchronous machines, with a rotor producing a permanent magnetic flux. The invention also relates to the field of sensorless rotor position determination for synchronous machines.
For the electronic commutation of polyphase synchronous machines it is necessary to know the rotor position of the rotor of the synchronous machine in order to apply a suitable, rotor-position-dependent phase voltage to the respective winding phase of the synchronous machine. Conventionally, the phase voltage or the phase current is applied as a constant voltage or constant current as long as the rotor is located within a specific rotor position range, in particular when a rotor is within an angular position range of an electrical rotor position.
It is often the case with brushless DC motors or permanent magnet synchronous machines that the rotor position is determined using complicated sensor technology. For this purpose, use is often made of Hall sensors or GMR (Giant Magnetic Resistance) sensors, which are arranged close to a rotor of the synchronous machine and provide an electrical signal as a measure of the rotor position. Such sensors additionally arranged in the synchronous machine are generally susceptible to faults and represent additional complexity in the manufacture of synchronous machines.
There is therefore an increasing trend in favor of the use of sensorless methods for rotor position determination. In sensorless methods, evaluation of the current profile through the synchronous machine is conventionally performed. This is generally imprecise since interference signals are generally superimposed on the current profile in the synchronous machine. This is primarily the case during operation at low speeds and during on-load startup. Therefore, sensorless methods for rotor position determination can generally only be used to a limited extent.
It is also possible to determine the rotor position of the synchronous machine by measuring the inductance of the stator coil. The inductance of the stator coil varies depending on the rotor position owing to the saturation in the stator coils brought about by the rotor magnets. The dependence of the inductance of the stator coil is a consequence of the superimposition of the magnetic field brought about by the permanent magnets and the magnetic field brought about by the measurement pulse, which magnetic fields can be added to one another or cancel one another out, depending on the rotor position. In the case of additive superimposition of the magnetic fields, the stator coil enters saturation and the inductance thereof thereby decreases. This inductance is measured by a measurement pulse on the stator coil, with this measurement pulse preferably being applied when the stator coil in question is in the deenergized state in order to avoid firstly influences of the measurement pulse on the formation of torque and secondly reaction on the measurement of the instantaneous inductance.
The accuracy of the measurement of the rotor-position-dependent inductance requires that the stator coil is deenergized since otherwise an erroneous measurement of the inductance would result from the superimposition of the magnetic field produced by the drive current through the stator coil owing to the effect of the magnetic saturation. The rotor position therefore cannot be determined precisely.